Move constructTextRun to RenderBlockFlow

RenderBlock should have no concept of TextRuns.

BUG=302024

Review URL: https://codereview.chromium.org/52893004

git-svn-id: svn://svn.chromium.org/blink/trunk@160999 bbb929c8-8fbe-4397-9dbb-9b2b20218538